1. Field of the Invention
The present invention relates to an industrial robot including a rotary arm for welding or the like.
2. Description of the Related Art
So-called articulated robots and other robots are, for example, used as industrial robots such as welding robots. Industrial robots such as welding robots are faced with a growing demand to perform welding in narrow places. Accordingly, downsizing of a tool mounting portion, to which a tool is to be mounted, and its vicinity is hoped to avoid interference with surrounding objects such as a work. Thus, it is necessary to optimize a driving mechanism of a tool mounting portion and a driving mechanism of an arm to which the tool mounting portion is to be mounted.
As a technology on a structure of a tool mounting portion of an articulated robot, to which a tool is to be mounted, there has been proposed a structure for avoiding interference with surrounding objects such as a work by changing a mounted position of a motor for driving and rotating an end effector relative to an end effector mounting portion (see, for example, Japanese Unexamined Patent Publication No. 2006-26748, hereinafter referred to as patent literature 1).
For an industrial robot, there has been also proposed a structure for avoiding interference with surrounding objects such as a work by vertically symmetrically arranging two sets of gear sets, which have symmetric twist directions commonly using two axis lines supported only on one end to downsize a wrist structure (see, for example, Japanese Patent Publication No. 4233578, hereinafter referred to as patent literature 2).
Further, for an industrial robot, there has been proposed a structure for avoiding interference with surrounding objects such as a work by arranging a reduction mechanism for pivoting motions on a pivot shaft in a robot including an obstruction-free and continuous passage for allowing passage of a power line, pipe arrangement and signal line for an end effector in an arm to downsize a wrist structure (see, for example, Japanese Unexamined Patent Publication No. H05-131388, hereinafter referred to as patent literature 3).
However, in the industrial robot disclosed in patent literature 1, a motor 603 for driving and rotating a tool 601 is mounted and arranged in a tool mounting portion 602 to which the tool 601 is mounted as shown in FIG. 7. Thus, if the arm 604 having the tool mounting portion 602 mounted thereto is pivoted relative to an arm 605 in directions of arrows X as shown in FIGS. 7A and 7B, a cable 606 connected to the tool 601 mounted to the tool mounting portion 602 interferes with the motor 603.
Further, as shown in FIG. 7C, the motor 603 interferes with a work object 607 when the tool 601 is inserted into a narrow place of the work object 607. At this time, since the mounted position of the motor 603 is changed according to an operation in the robot disclosed in patent literature 1, operation efficiency decreases.
In an industrial robot, if a distance between a tool mounting portion 612 having a tool 611 mounted thereto and a rotation axis A11 of an arm 613 having the tool mounting portion 612 mounted thereto relative to an arm 614 is shortened as shown in FIG. 8A, a bend radius of a cable 615 connected to the tool 611 decreases and a load acts on the cable 615. Thus, failures occur in the feed of a welding wire and supply of welding gas, for example, in a welding robot or the like, wherefore a welding failure occurs.
Thus, in the industrial robot, it is desired to extend the distance between the tool mounting portion 612 having the tool 611 mounted thereto and the rotation axis A11 of the arm 613 having the tool mounting portion 612 mounted thereto relative to the arm 614 as shown in FIG. 8B.
However, the industrial robot disclosed in patent literature 2 is structured to drive and rotate a tool mounting portion 612 mounted to an arm 613 via a bevel gear 621 as shown in FIGS. 9A and 9B. Thus, if a distance from the tool mounting portion 612 to a rotation axis A21 is long, a diameter of the bevel gear needs to be increased to engage the bevel gear 621 with the tool mounting portion 612. If the diameter of the bevel gear 621 increases, the vicinity of the arm 613 becomes larger. This deteriorates application efficiency to a work and increases a gear ratio, wherefore positioning accuracy of a tool 611 is deteriorated.
Further, if the diameter of the bevel gear 621 is reduced as shown in FIGS. 10A and 10B in the industrial robot disclosed in patent literature 2, a bend radius of a cable 615 decreases and a load acts on the cable 615 when the arm 613 is rotated.
Further, in the industrial robot disclosed in patent literature 3, two reduction mechanisms, i.e. one for pivoting the arm and the other for rotating the tool mounting portion need to be arranged on the pivot shaft of the arm. Thus, the thickness of the arm increases to enlarge the arm. Further, since the reduction mechanism is directly mounted on the arm in the structure disclosed in patent literature 3, assembling and maintenance cannot be easily performed.